The present invention relates to a process for producing top anti-reflective coatings for photoresists, which coatings have a very low level of metal ions, especially sodium and iron, and for using such top anti-reflective coatings with light-sensitive photoresist compositions to produce semiconductor devices. Further, the present invention relates to a process for coating substrates already coated with a photoresist composition with these top anti-reflective coating compositions, as well as the process of coating, imaging and developing light-sensitive photoresist compositions coated with such anti-reflective coatings on such substrates.
Thin film interference plays a central role in the process control of optical microlithography. Small variations in the thickness of resist or of thin films underneath the resist cause large exposure variations, which in turn cause two classes of undesirable line width variations.
1. As thin film thickness may vary from run to run, wafer to wafer, or across a wafer, line widths will vary from run to run, wafer to wafer or across a wafer. PA1 2. As patterning takes place over wafer topography, the resist thickness unavoidably changes at the topography edge causing the line width to vary as it crosses the edge.
Avoiding such thin film interference effects is one of the key advantages of advanced processes such as X-Ray lithography or multi-layer resist systems. However, Single Layer Resist (SLR) processes dominate semiconductor manufacturing lines because of the their simplicity and cost-effectiveness, and also because of the relative cleanliness of wet developed processes compared with dry processes.
Thin film interference results in periodic undulations in a plot of the exposure dose required to clear positive photoresist (termed dose-to-clear) versus the photoresist thickness. Optically, on a resist-coated substrate, light reflected from the bottom mirror (due to the effect of the substrate+thin films) interferes with the refection of the top mirror (the resist/air interface).
As optical lithography pushes towards shorter wavelengths, thin film interference effects become increasingly important. More severe swings in intensity are seen as wavelength decreases.
One strategy for reducing thin film interference is to reduce the substrate reflectivity through the use of absorptive Anti-Reflective Coats. One way of doing this is to apply a Top Anti-Reflector coating on top of the photoresist prior to exposure.
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist and all of the anti-reflective coating from the surface of the substrate.
Metal contamination has been a problem for a long time in the fabrication of high density integrated circuits and computer chips, often leading to increased defects, yield losses, degradation and decreased performance. In plasma processes, metals such as sodium and iron, when they are present in the photoresist or in a coating on the photoresist, can cause contamination especially during plasma stripping. However, these problems have been overcome to a substantial extent during the fabrication process, for example, by utilizing HCL gathering of the contaminants during a high temperature anneal cycle.
As semiconductor devices have become more sophisticated, these problems have become much more difficult to overcome. When silicon wafers are coated with a liquid positive photoresist and subsequently stripped off, such as with oxygen microwave plasma, the performance and stability of the semiconductor device is often seen to decrease. As the plasma stripping process is repeated, more degradation of the device frequently occurs. A primary cause of such problems can be the metal contamination in the anti-reflective coating on the photoresist, particularly sodium and iron ions. Metal levels of as low as 1.0 ppm or less can adversely affect the properties of such semiconductor devices.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating. Thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble in the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble in the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. The etchant solution or plasma gases etch that portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive working photoresist compositions are currently favored over negative working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less then one micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.